The present application relates to a thin film transistor (TFT) using oxide semiconductor, a method of manufacturing the same, and a display unit including such a thin film transistor.
Oxide semiconductor composed of zinc oxide, indium gallium zinc oxide (IGZO) or the like shows superior characteristics as an active layer of a semiconductor device. In recent years, development has been promoted in an effort to apply the oxide semiconductor to a TFT, a light emitting device, a transparent conducting film or the like.
For example, in the TFT using the oxide semiconductor, electron mobility is high and its electric characteristics are superior compared to the existing TFT using amorphous silicon (a-Si:H) as a channel that is used for a liquid crystal display. Further, the TFT using the oxide semiconductor has an advantage that high mobility is expected even if the channel is formed at a low temperature around room temperature.
Meanwhile, it has been known that in the oxide semiconductor, the heat resistance is not sufficient, and thus due to heat treatment in a manufacturing process of the TFT, oxygen, zinc and the like are detached and lattice defect is formed. The lattice defect results in forming an electrically shallow impurity level, and causes low resistance of the oxide semiconductor layer. Thus, in the TFT using the oxide semiconductor as an active layer, it results in normally-on type operation or depression type operation in which a drain current is flown without applying a gate voltage, the threshold voltage is decreased as the defect level is increased, and the leakage current is increased.
Meanwhile, in addition to the foregoing lattice defect, hydrogen has been reported as an element to form shallow impurity level (for example, see “n-type doping of oxides by hydrogen,” Cetin Kilic et al., Applied Physics Letters, Jul. 1, 2002, Vol. 81, No. 1, pp. 73 to 75). Thus, in addition to the lattice defect, an element such as hydrogen introduced in manufacturing process of the TFT has been regarded as a substance that affects characteristics of the TFT using the oxide semiconductor. Accordingly, in the transistor having the oxide semiconductor as a channel, carrier concentration in the channel tends to be increased, and the threshold voltage tends to be negative.
Further, in the TFT using the oxide semiconductor as a channel, it is difficult to form P-channel. Thus, the circuit should be formed from only an N-channel transistor. At this time, there is a disadvantage that if the threshold voltage becomes negative, the circuit configuration becomes complicated. To solve such a disadvantage, the threshold voltage should be controlled. The threshold voltage is expressed by the following mathematical formula.
                              V          Th                =                              ϕ            MS                    -                                    Q              f                                      C              OX                                +                      2            ⁢                                                  ⁢                          ϕ              f                                +                                                    2                ⁢                                                                  ⁢                                  ɛ                  S                                ⁢                                  ɛ                  0                                ⁢                                  qN                  A                                ⁢                2                ⁢                                                                  ⁢                                  ϕ                  f                                                                    C              OX                                                          Mathematical        ⁢                                  ⁢        formula            
In the formula, Vth represents the threshold voltage, ΦMS represents work function difference between a gate electrode and an oxide semiconductor film, Qf represents fixed charge, COX represents gate insulating film capacity, Φf represents Fermi level of the oxide semiconductor film as a channel, NA represents acceptor density, ∈S represents relative permittivity of the oxide semiconductor film, and ∈0 represents dielectric constant in vacuum, respectively.
As a method of changing the threshold voltage, trials to change the threshold voltage have been made by doping impurity in part of a channel in an interface of a thin film transistor and a gate insulating film, or by changing ratio of elements of oxide semiconductor (for example, see Japanese Unexamined Patent Application Publication Nos. 2007-519256 and 2008-85048).